In a common wind farm control structure, a farm controller (high level controller) is located at the Point of Common Coupling (PCC), i.e. the connection between wind farm and a public grid. As the control structure in the common wind farm control structure is hierarchical, the lower level controllers are located at individual turbines, the so called turbine controllers.
The farm controller has the task to regulate the overall wind farm active power generation and additional variables, for example the voltage, the reactive power or the power factor at the PCC. The farm controller receives current and voltage measurements taken at the PCC, and based on their difference from the corresponding reference values, it provides the voltage set-points for the individual turbines. An implementation of the farm controller can be a PID structure with a possible droop function modifying the reference voltage based on the measured reactive power and possibly modifying the active power reference based on the measured grid frequency.
The turbine controllers regulate a terminal voltage and a turbine active power production, e.g. following the maximum power point tracking, defined by the reference values, i.e. the set-points, defined by the farm controller. This is effectively done by producing the set-points for the Id (based on the active power set point) and Iq (based on the voltage or reactive power set point) current components which are then, if not out of range, generated by the converter.
In such a structure, the farm controller provides the same voltage and often the same active power set-points to all turbine controllers without taking into account their individual situation, such as the grid circuit characteristic, the individual turbine wind exposure, reactive or active power capabilities of the turbines, saturation status etc. Further, the farm controller does not consider the current saturation limits of the turbine controllers, that means of their converters, which might lead to large integral values in the corresponding turbine PI controllers. Further, since such a structure uses hierarchical control loops, the stability analysis of the overall farm control scheme may be very complex since the turbine controller dynamics cannot be neglected with respect to the time constant of the farm controller.